Chemistry - Answers (Give Reason )

Answer

1 - Size of the atoms of inert gases are bigger because the outer shell of inert gases is complete and have maximum number of electrons in their outer most orbit thus the electronic repulsion are maximum.

2 - Group 17 elements have a strong tendency to gain electron ,in order to attain octet configuration so they are strong non - metal, where as group 1 have a tendency to lose electron .

3 - On moving across a period , nuclear pull increases due to the increase in atomic number, and thus the atomic size decreases. Hence ,elements cannot lose electrons easily.Where as on moving down a group ,the atomic size and the nuclear charge also increases.

4 - Halogens have high electron affinity because they are smaller in size and have greater the nuclear charges compared to other elements.They have a strong tendency to accept an additional electron .

5 - Reducing power depends upon the ease of ejection of electron from an atom. As we move down the group , effective nuclear charge decreases ,therefore electron can be easily ejected so reducing power increase on moving down in a group.

6 - On moving from left to right in the third period ,the atomic number of elements increases that means number of protons and electrons in the atoms increases but the number of shells remain the same for all the elements of a given period . The extra electron enter to the same shell . Due to increase in positive charge on the nucleus , the electron in the outer most shell are pulled towards the nucleus .As a result size of atom decreases from Na to Cl.

7 - The size of Cl ion is greater because Cl ion has one electron more and more orbit compared to Cl atom.

8 - The size of Argon is greater than the size of Cl because of the inter-electronic repulsion start taking place when an atom attain its octate.

9 - Ionization potential is the energy required to remove an electron from a neutral gaseous atom ,which means on moving across a period because of the atomic size decreases due to an increase in the nuclear charge more energy required to remove the electrons.

10 - The atoms of alkali metals do not have a strong attraction for the outermost single valence electron, so they are easily donate electron . So alkali metals are good reducing agent.

11 - Nitric acid is a very strong oxidizing agent so it is not used in the preparation of hydrogen.

12 - Covalent compounds are volatile in nature because they have the weak forces of attraction between the binding molecules,thus less energy is required to break the force of bonding ,so they have low boiling and melting point.

13 - As water is a polar compound ,it decreases the electrostatic forces of attraction resulting free ions in aqueous solution ,Hence they dissolve electrovalent compounds .Where as organic solvents are non- polar compounds, Hence these dissolve in non- polar covalent compounds.

14 - Electrovalent compounds contain oppositely charged ions which are tightly bonded by electrostatic force of attraction ,so they posses the close packed structure .Thermal energy of the ions is low which keeps them bonded in a crystal lattice structure ,thus electrovalents compounds are hard.They are brittle because when strong force is applied ,the energy supplied by force displaces the layer of the ionic solids which cause electrostatic repulsion between the layers.

15 - In the special condition polar covalent compounds conduct electricity.When they are dissolved in water (aqueous solution ) they conduct electricity as the electrons are free to move in solution rather than in a solid.

16 - Hydrochloric acid has only one hydrogen atom that is displaced during the reaction therefore hydrochloric acid salts are not acidicin nature On the other hand carbonic acid is diprotic and on neutrilization it still contains one hydrogen atom that can be displaced, tharefore carbonic acid salts are acidic in nature.

17 - HCl acid ionises completely in H + and Cl- .Hence it is a strong acid where as degree of ionisation is low for concentrated acetic acid.

18 - H3PO3 is not a tribasic acid .It is adibasic acid because in oxyacid of phosphorous, hydrogen atoms which are attached to oxygen atom are replaceable. Hydrogen atom directly bonded to phosphorus atom are not replaceable.

  

19 - Lead carbonate does not react with HCl or Sulphuric acid because the salt produced after reaction is insoluble.

20 - NO2 is double acid anhydride because two acids ,nitrous oxide and nitric acid , are formed when it react with water.

21 - Since sodium hydroxide and sulphuric acid are both soluble , an excess of either of them can not be removed by filtration.Therefore it is necessary to find out the ratio of reactants.

22 - FeCl3 (III) is highly deliquescent ,so kept it dry fused with calcium chloride (drying agent).

23 - Anhydrous FeCl3 can not be prepared by simply heating the hydrated Iron (III) chloride because on heating anhydrous FeCl3 produces Fe2O3.H2O and HCl.

24 - Common salt get wet during rainy season because it is deliquescent in nature. When it exposed to the atmosphere at ordinary temperature ,absorb moisture from the atmospheric air to become moist.

25 - Hydrogen gas is diatomic (H2). where as Helium gas is monoatomic (He) . Therefore volume of hydrogen is twice at the same temperature and pressure.

26 - Rate of change of volume of gas depends on the rate of change of temperature and pressure .According to ideal gas equation PV=RT gas expandsas temperature and pressure change .Volume is measured how much space the gas has occupied, so if the temperature and pressure changes the gas will expand or come closer together .which effect the volume of that gas.

27 - When air is blown into a balloon , the volume and pressure inside the balloon increases . According to Boyel's law the volume of given mass of dry gas is inversely proportional to its pressure at a constant temperature.So law is not valid here.

28 - Carbon - 12 was selected as a unit , because its adaptation least affected the values of the atomic mass of the various elements on the old standard.

29 - A solution of sugar cane does not conduct electricity , because there are no ions present, where as in the aqueous state of sodium chloride, the slightly negatively charged oxygen atoms of the polar water molecule exert a pull on the positively charged sodium ions. A similar pull is exerted by the slightly positively charged hydrogen atoms of the water on the negatively charged chloride ions. Thus the ions becomes free in solution and electricity can be conduct.

30 - Water is a polar solvent ,when HCl is added to water ,the slightly negatively charged oxygen atom of the water exert an electrostatic pull on slightly positively charged hydrogen ions present in the molecules of HCl. Than HCl conduct electricity

32 - On electrolysis of dilute copper (II) sulphate solution , copper is deposited at the cathode not hydrogen because in activity series copper ion is below hydrogen ion.

33 - When dilute aqueous solution of sodium chloride is electrolysed hydrogen gas evolved at the cathode because dispite of metallic sodium hydrogen gas gains electron more easily .So hydrogen is evolved at cathode.

34 - Zinc reacts with hydrochloric acid to produce hydrogen gas because its more reactive than hydrogen,and thus displaces the latter from an acid .By comparison copper can not displace hydrogen from HCl because it is less reactive than hydrogen.

35 - In the electrolysis of acidified water dilute sulphuric acid is used because it is non - volatile while nitric acid is volatile acid.

36 - When electricity is passed through molten lead bromide ,it decomposes to Pb 2+ ions and Br - ions .The Pb 2+ ions move towards cathode and gain two electrons to form lead atom.So Pb 2 + ions reduced . side by side,the Br - ions move towards anode and loose electrons to form bromine gas. Thus bromine ions are oxidized. Hence electrolysis of lead bromide is a redox reaction.

37 - The products of the electrolysis of copper sulphate solution using platinum electrodes are first copper deposit on cathode electrode second oxygen gas at anode. As more and more copper is deposited , the blue color of the solution fades as the concentration of blue copper ions in solution is consumed in the reaction.

38 - Lead bromide consists of Pb 2+ and Br- .In solid state ,these ions bonded in a lattice crystalline structure by strong electrostatic forces of attraction.these forces do not allow the ions to dissociate and hence lead bromide does not conduct electricity and is a non- electrolyte. In a molten state , the ions are free and therefore capable of conduct electricity.

39 - Normally carbon is used as a reducing agent during extraction of metals . However Aluminium is a strong reducing agent and can not be reduced by carbon . Also aluminium oxygen bond is extremely strong and requires high energy to break which is difficult to maintain hence electrolytic reduction method is conventional.

40 - At the anode OH- loses one electron and becomes neutral OH.The combination of OH forms water .

 2OH +2OH →2H2O + O2

 now 4 OH neutral particles are involved in equation so 4 electrons are lost to get 4OH neutral particles

 4OH- → 4OH + 4e-

 Thus, the formation of 1 molecule of oxygen at anode releases 4 electrons. At cathode these 4 e- take up

 4H+ 4e- →2H2

 this show,electrolysis of water gives 2:1 by volume at the cathode and anode.

41 - Ammonia is a covalent compound in the gaseous state . In aqueous state ,its forms ammonium hydroxide which is a weak electrolyte.

42 - During the electrolysis of lead bromide graphite anode is preferred to other inert electrodes because graphite is unaffected by the reactive bromine vapours which liberate on anode.

43 - Silver nitrate is not used because ,when silver nitrate is used then the deposition of silver is very rapid and electroplating is not uniform due to very fast reaction.

44 - Carbon tetrachloride is a covalently boned compound there is no passage for conduct electrcity .it is not necessary all liquids conduct electricity.Carbon and chlorine both are non conducting elements present in CCl4.

45 - Alkali metals are kept in kerosene oil as they react with moisture and air.

46 - The reduction of dilute acids with lead is very slow, because the salts lead chloride and lead sulphate formed are sparingly soluble in dilute acids. During the reaction , the chloride or sulphate gets coated on metal and prevent reaction to procced further, so rate of reaction decreases.

47 - Most of the metals are reactive so they occur in combine state.

48 - The highly reactive metals can be used as reducing agent because they can displace metals of lower reactivity from their compounds.

49 - Dilute or concentrated nitric acid does not attack the metal aluminium.

50 - In the electrolytic reduction of alumina, the graphite anode has to be replaced time to time ,as it gets oxidized to carbon monoxide ,which further forms CO2.

51 - Roasting is carried out on sulphide ores because it converts sulphide ores into oxide .When carbonate ores is heated in absence of air it is high but not sufficient to melt the air.

52 - In activity series Aluminium is above the lead and it has a great affinity towards oxygen so can not be reduced by carbon.

53 - The ore of aluminium (Aluminium oxide ) due its great affinity for oxygen is a very stable compound . it is not reduced easily by common reducing agents Hence ,electrolytic reduction is done to obtain aluminium.

54 - Zinc ,being one of the metals with a really slow corrosion rate ,is a fairly feasible rust preventing coating .When zinc is scratched and iron exposed to air, iron does not rust because of the remaining parts of the sacrificial anode that is zinc.

55 - The reaction occurring at anode is :

 2O-2 + 4 e- →O2

 As O2- is released at anode which is made up of carbon, the other gas which formed is CO2

56 - Nitric acid reacts with aluminium to form a thin layer to form a thin protective aluminium oxide layer on the aluminium surface . This oxide layer renders aluminium passive and prevents it to further react in nitric acid and thus the conc. nitric acid is not affected by the container.